The present application claims priority to Japanese Application No. P11-176007 filed Jun. 22, 1999 which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery incorporating a positive-electrode active material, a separator, a negative-electrode active material and an electrolytic solution.
2. Description of the Related Art
A nonaqueous electrolyte secondary battery capable of enduring heavy load discharge and thus permitted to repeatedly be used owing to charge has widely been used as a power source for a variety of portable electronic apparatus pieces. Since reduction in the size and weight of the electronic apparatus has been realized, also the nonaqueous electrolyte secondary battery which is the power source for the portable electronic apparatus has been required to have reduced size and weight and a high energy density.
In particular, a lithium ion secondary battery has been employed as the nonaqueous electrolyte secondary battery which is capable of satisfying the foregoing requirements.
As a material which can be used as the positive-electrode active material of a 4-volt lithium ion secondary battery, a lithium-cobalt oxide, a lithium-nickel oxide and a lithium-manganese oxide are known materials. From a viewpoint of realizing safety and obtaining a high energy density, a lithium-cobalt oxide is a preferred material.
A separator for a nonaqueous electrolyte secondary battery, such as the lithium-ion battery, is constitutes by a polyolefin small-pore film represented by polymer polyethylene and polymer polypropylene. A primary battery frequently incorporates a separator containing cellulose, such as paper or unwoven fabric.
A small-pore polyolefin battery having proper gas permeability is melted when the internal temperature of the battery is heated to about 120xc2x0 C. to about 170xc2x0 C. Thus, pores in the small pores are closed. As a result, a shutdown effect occurs with which movement of lithium ions is blocked and any electric current flows. The shutdown effect is used as a safety means to prevent flow of any excess current caused from runaway of chemical reactions in the battery.
The lithium-cobalt oxide which is employed as the positive-electrode active material of the lithium-ion secondary battery suffers from a problem in that the places of origin of cobalt are limited and cost of cobalt, which is rare metal, is instable.
The small-pore polyolefin film for use in the separator of the nonaqueous electrolyte secondary battery which requires a complicated manufacturing process encounters a problem in that the cost cannot be reduced.
The temperature at which oxygen is discharged from the lithium-cobalt oxide is lower than that of a spinel lithium-manganese composite metal oxide by about 130xc2x0 C. Therefore, there is apprehension that the small-pore polyolefin film is melted and discharged when the temperature of the battery has been raised to a level higher than 130xc2x0 C. in a case where the temperature of the battery is raised owing to preservation at high temperatures or external short circuit. In the foregoing case, physical contact between the positive electrode and the negative electrode sometimes causes short circuit to occur.
The characteristics of the separator of the nonaqueous electrolyte secondary battery as the film greatly concern the characteristics of the battery. The characteristics as the film can be expressed by the film thickness and the permeability. The xe2x80x9cpermeabilityxe2x80x9d is a degree of air penetration of paper. That is, the degree is expressed by time required for air in a predetermined quantity (assumed to be 100 cc) to pass through a test piece under predetermined condition, the time being expressed by seconds. Therefore, air quickly passes through the film as the foregoing value is reduced. Thus, the pressure which is applied to the film is lowered. In the foregoing case, the permeability is enlarged. As the value is enlarged, penetration of air takes a long time, causing the pressure which is applied to the film to be raised. Thus, the permeability is reduced.
In general, the characteristics of the battery is improved as the film thickness is reduced and the permeability is enlarged. A separator having a great permeability has a low resistance thereof, causing the characteristics of the battery required when a high load to be improved. On the other hand, a frequency of occurrence of short circuits is raised undesirably, causing the manufacturing yield to deteriorate. A separator having small permeability is caused to have high resistance. As a result, there arises a problem in that the battery cannot be operated.
In view of the foregoing, an object of the present invention is to provide a nonaqueous electrolyte secondary battery incorporating a positive-electrode active material and a separator which are made of materials which are relatively easily available and exhibiting excellent heat resistance and satisfactory characteristics as the battery.
To achieve the foregoing object, according to one aspect of the present invention, there is provide a nonaqueous electrolyte secondary battery incorporating a positive electrode containing a positive-electrode active material and a negative electrode containing a negative-electrode active material which are laminated through a separator and containing nonaqueous electrolytic solution enclosed therein, the nonaqueous electrolyte secondary battery including: a spinel manganese composite metal oxide serving as the positive-electrode active material, wherein the separator is constituted by paper having a thickness of 15 xcexcm to 60 xcexcm and permeability of 1 second/100 cc to 10 seconds/100 cc.
The nonaqueous electrolyte secondary battery according to the present invention incorporates the spinel manganese composite metal oxide serving as the positive-electrode active material. The spinel manganese composite metal oxide barely contains cobalt, which is rare metal, so that it is readily available and advantages of manufacturing cost.
Furthermore, the spinel manganese composite metal oxide discharges oxygen at a temperature which is higher than that of the lithium-cobalt composite metal oxide by about 130xc2x0 C. Therefore, any shutdown function is required. As a result, a paper separator can be employed.
Paper can be manufactured without any complicated manufacturing process required for a small-pore polyolefin film. Moreover, paper exhibits excellent heat resistance. When the thickness and the permeability of paper are properly determined, characteristics of the battery required in a case of a high load can be maintained. Moreover, unintentional short circuit can be prevented.